- Improve VBOXVD_DESC, define LICENSE, remove redundant PKGMESSAGE
- Simplify post-install recipe, generally cleanup Makefile while here
- Convert PLIST_* knobs into pkg-plist (8 lines make it long enough
  to be kept in a separate file), use sample@ keyword


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@374784 35697150-7ecd-e111-bb59-0022644237b5